If an object was changed to not export symbols anymore, the
corresponding stale .ver file would have been left lying around
and been picked up when generating modversions.h. 

The obvious solution to remove include/linux/modules/* at the
beginning of "make dep" is not really good, since that means
that .ver files would be regenerated unconditionally, thus causing
a lot of possibly unnecessary rebuilds.

So, instead, we build a temporary shadow tree of all export-objs
(as empty files) during the recursive "make fastdep" phase, and use 
that to generate modversions.h.

Ensure that we touch include/linux/modversions.h if any of the
.ver files changes, that's our marker to rebuild all modversions
affected files.